This invention relates to a method of storing optical information, and particularly to such a method in which the optical information can be quickly stored without the use of any external electrical power and in which the stored optical information can be erased electrically.
Electrochromic devices are devices which exhibit a phenomenon known as "persistent electrochromism," e.g., see U.S. Pat. No. 3,521,941 entitled, "Electro-Optical Device Having Variable Optical Density," issued July 28, 1970. The term persistent electrochromism denotes the property of a material whereby its electromagnetic radiation absorpiton characteristic is altered under the influence of an electric field. By persistent it is meant the ability of the material to remain in the absorptive state after the removal of the electric field. Such materials, for example, may exhibit little or no absorption of visible wavelength in the absence of an electric field, and therefore be transparent, but when subjected to an electric field, effectively absorb in the red end of the spectrum, turning blue in color.
The phenomenon of persistent electrochromism has been exhibited in electrochromic devices which include an electrolyte-electrochromic sandwich wherein the electrolyte functions both as a conductive medium and as a source of positive ions. For example, see the sulfuric acid electrolyte of U.S. Pat. No. 3,708,220, issued Jan. 2, 1973. In these devices, the electrolyte is chosen to be sufficiently conductive so as to permit low voltage operation of the electrochromic device while also being chemically compatible with the electrochromic layer and electrode employed in the device.
Although the aforementioned U.S. Pat. No. 3,521,941 indicated that many of the electrochromic materials also exhibit the property of photochromism under ultraviolet light, it has been observed that methods of storing optical information which utilize the photochromic properties of electrochromic materials are time consuming, i.e., the materials are generally slow to color.
Another method of storing optical information which has been disclosed includes the use of a photoconductor layer in an electrochromic device such that the device becomes photoresponsive to the optical information to be stored. In this method, coloration speed, i.e., storage speed, is adequate. For example, coloration times of a few second are typical. Also, in this method, the photoconductor layer can be matched to the optical input. However, this method still requires the use of electrical energy, i.e., a biasing voltage in order for coloration to occur. The biasing voltage is required to be of a magnitude such that the electric field provided is insufficient to initiate the coloration process in the absence of the optical information. That is, in this method, the photoconductor layer functions as a switch.
Thus, it would be desirable to develop a method of storing optical information which is capable of fast coloration, i.e., fast storage, and in which the specific kind of optical information required for coloration can be chosen independently of the photochromic sensitivity of the electrochromic material. Also, it would be desirable to develop a method of storing optical information in which a biasing voltage is not required for storing the optical information.